Question

Changing R default library path using libPaths

Answer 1

You should try using one library but if at all you feel like changing it then why don't you try appending the new library to the existing library? 
You can do so as follows:

.libPaths( c( .libPaths(), "~/usersLib") )
 

Then:

> .libPaths()
[1] "/Library/Frameworks/R.framework/Versions/2.15/Resources/library"
[2] "/Users/davidwinsemius/usersLib"